Lithographic apparatus and device manufacturing method

ABSTRACT

Provided is a method and system for facilitating use of a plurality of individually controllable elements to modulate the intensity of radiation received at each focusing element of an array of focusing elements to control the intensity of the radiation in the areas on the substrate onto which the focusing elements direct the radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/593,041,filed Nov. 6, 2006, now U.S. Pat. No. 7,522,266, which is a divisionalof application Ser. No. 10/779,811, filed Feb. 18, 2004, now issued asU.S. Pat. No. 7,133,118. The entirety of each of the foregoingapplications is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. A lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs), flatpanel displays and other devices involving fine structures. In aconventional lithographic apparatus, a patterning means, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern corresponding to an individual layer of theIC (or other device). This pattern can be imaged onto a target portion(e.g., comprising part of, one or several dies) on a substrate (e.g., asilicon wafer or glass plate) that has a layer of radiation sensitivematerial (resist). Instead of a mask, the patterning means may comprisean array of individually controllable elements which serve to generatethe circuit pattern.

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion in one go, and socalled scanners, in which each target portion is irradiated by scanningthe pattern through the projection beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

In the manufacture of flat panel displays, for example, it is oftendesirable to be able to expose the substrate such that different regionsof substrate receive different overall doses of radiation. By so doingit is possible to completely remove a resist on the substrate in someregions, leave the full thickness of resist in other regions and alsoprovide additional regions in which the resist has been partiallyremoved, for example.

This capability is often referred to as half tone or gray tone exposure.This enables a plurality of processing steps in the subsequentmanufacturing process to be performed for a single exposure step. Forexample, a process may be applied to the parts of the substrate that arefully exposed after the exposure. Next a given thickness of resist isremoved from those regions which are still covered by resist. Thisexposes additional regions of the substrate but does not expose thoseregions where the resist was thickest; subsequently an additionalprocessing step can be performed on only those regions that are nowexposed. Finally, all of the remaining resist may be removed before aprocessing step is performed on the entire substrate.

Gray tone exposure may be produced by individual portions of thesubstrate being exposed at a given intensity for different amounts oftime, by exposing individual portions for the same amount of time but atdifferent intensities (the capability for which is commonly referred toas gray scale exposure) or by a combination of the two.

The present invention provides a method and apparatus for applyingdifferent radiation doses to different regions of a substrate.

SUMMARY OF THE INVENTION

Consistent with the principles of the present invention as embodied andbroadly described herein, a lithographic apparatus includes anillumination system for supplying a projection beam of radiation. Alsoincluded is an array of individually controllable elements serving toimpart the projection beam with a pattern in its cross-section and asubstrate table for supporting a substrate initially, a projectionsystem is included for projecting the patterned beam onto a targetportion of the substrate, wherein the projection system includes anarray of focusing elements, arranged such that each focusing elementdirects the radiation in the patterned beam from a plurality of theindividually controllable elements onto an area on the substrate.

Accordingly, if all of the individually controllable elements associatedwith a given focusing element are set to provide high intensityradiation to the focusing element, the intensity of the radiation in theassociated area on the substrate will be high and the dose of radiationreceived over a given exposure time will be relatively high. If all ofthe individually controllable elements associated with the focusingelement are set such that a low intensity of radiation (or, preferably,no radiation) is directed to the focusing element, then the intensity ofthe radiation (and hence the radiation dose for a given exposure time)in the area on the substrate will be low.

By setting some of the individually controllable elements associatedwith the focusing element to direct high intensity radiation to thefocusing element and some to direct low intensity radiation to thefocusing element, the intensity of the radiation in the associated areaon the substrate will be at an intermediate value, thereby providing anintermediate dose of radiation for the same exposure time as in theprevious two settings. Accordingly, by providing gray scale control, therequired gray tone control can be provided. It will be appreciated thatthe more individually controllable elements there are associated witheach focusing element, the greater the number of possible intermediatelevels of radiation intensity, or gray scale levels, that can beprovided in each area on the substrate associated with each focusingelement.

Preferably, each of the individually controllable elements can be set tothree or more states. For example, a first state is provided in whichthe maximum proportion of the intensity directed onto the individuallycontrollable element is directed onto the associated focusing element. Asecond state is provided in which a minimum, preferably substantiallyzero, proportion of the radiation incident on the individuallycontrollable element is directed to the corresponding focusing element.Additional states are provided in which the proportion of the radiationincident on the individually controllable element that is directed tothe corresponding focusing element is between that of the first andsecond states and different to any other states. By this means,additional gray scale levels can be provided.

According to another preferred embodiment, each of the individuallycontrollable elements may be configured such that the proportion of theradiation incident on it that is directed to the associated focusingelement in each of its states is different to that of each of the otherindividually controllable elements associated with that focusingelement. This enables yet further gray scale levels to be provided. Forexample, consider an arrangement with three individually controllableelements associated with one focusing element. If the elements alldirect the same proportion of the incident radiation to the focusingelement in their maximum intensity state, then the intensity in the areaon the substrate illuminated by the focusing element would be the sameif any one of the individually controllable elements was set to themaximum state and the others set to a state in which zero radiation wasdirected to the focusing element. By arranging the maximum states foreach of the elements to be different from one another, these threesettings provide three different radiation intensities on the area onthe substrate illuminated by the focusing element and hence differentradiation doses or gray tones for a given exposure time.

A similar effect can be provided by having the same states for each ofthe individually controllable elements (i.e., in each of thecorresponding states, all of the individually controllable elementsdirect the same proportion of the incident radiation onto the focusingelement). At the same time, however, the radiation incident on eachindividually controllable element is attenuated such that the radiationincident on each associated element with a given focusing element isdifferent. Alternatively, the radiation propagating from each of theindividually controllable elements is attenuated such that a differentproportion of the radiation directed from each of the individuallycontrollable elements associated with a given focusing element reachesthe focusing element. Some combination of the above methods may also beused.

Preferably, the apparatus is configured so that, in total, each point onthe substrate associated with a single focusing element can be set toanyone of 256, 512 or 1024 gray scale levels of radiation intensity.

The apparatus may further include an actuator for moving the substraterelative to the projection system at a substantially constant velocitywhile a predetermined portion of the substrate is exposed. As thesubstrate scans beneath the projection system, the settings for theindividually controllable elements are changed to provide the requiredpattern. In a preferred arrangement, the apparatus may further comprisea controller, for providing the settings to the individuallycontrollable elements, which is arranged to be able to change thesettings of the individually controllable elements while a point on thesubstrate is within the area illuminated by one of the focusingelements. Therefore, during the time that one point on the substrate isexposed by each focusing element, two different settings can be appliedto the individually controllable elements. This provides additionalcontrol of the exposure dose received by such a point on the substrate.

For example, if the settings are changed halfway through the exposuretime of that point then the radiation dose received by that point willbe the average of the radiation dose that would have been received hadthe first setting been maintained for the full exposure time and theradiation dose that would have been received, had the second settingbeen maintained for the full exposure time. Therefore, if oneindividually controllable element is changed from full intensity to nointensity then the effect is to provide an exposure dose equivalent tothat individually controllable element having been at half intensity forthe full exposure. Thus, the effect of having an individuallycontrollable element with an intermediate setting can be re-created evenif it is not possible to provide such a setting to the individuallycontrollable elements. Similarly, if the individually controllableelements can be set to intermediate states the effect of additionalintermediate states can be created. Accordingly, additional gray tonesare provided.

In a similar fashion, increased control of the radiation dose can beprovided where each point on the substrate passes through two areasilluminated by different focusing elements. In this case, the controllercan provide different settings to the individually controllable elementsfor each of the two sub-exposures (i.e., the exposures received fromeach focusing element) for that point and the total dose will be the sumof the two sub-exposures. Therefore, analogous to the situationdescribed above, the total radiation dose will be equivalent to theaverage of the radiation dose that would have been received fromexposure at the first setting for two sub exposure times (i.e., the timefor the point to pass through the two illuminated areas) and theradiation dose that would have been received from exposure at the secondsetting for two sub exposure times. It will be appreciated that thistechnique can be combined with any of the preceding methods forcontrolling the total exposure dose, for example, those used to controlthe gray scale radiation intensity of the exposure.

According to yet another aspect of the present invention, there isprovided a device manufacturing method comprising the steps of providinga substrate and using an array of individually controllable elements toimpart the projection beam with a pattern in its cross-section. Alsoincluded are the steps of using an array of focusing elements as part ofa projection system to project the patterned beam onto a target portionof the substrate. Each of the focusing elements is arranged to directradiation in the patterned beam from a plurality of the individuallycontrollable elements onto an area within the target portion. Theindividually controllable elements can be set to a plurality ofdifferent states, in each of which a different intensity of radiationpropagates from the individually controllable element to the associatedfocusing element. The method further includes setting each of theindividually controllable elements to the required states to produce arequired intensity of radiation at said areas on the substrate.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.,water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts a schematic representation of a portion of the apparatusaccording to the present invention;

FIG. 3 depicts a substrate after an exposure;

FIG. 4 a depicts a substrate after a different exposure regime than thatof FIG. 3; and

FIG. 4 b depicts the substrate of FIG. 4 a after subsequent processingsteps.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the detailed description is not meant tolimit the invention. Rather, the scope of the invention is defined bythe appended claims.

It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of software, hardware, firmware, and/or the entitiesillustrated in the figures. Any actual software code with thespecialized control of hardware to implement the present invention isnot limiting of the present invention. Thus, the operational behavior ofthe present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

By way of background, the term “array of individually controllableelements” as here employed should be broadly interpreted as referring toany means that can be used to endow an incoming radiation beam with apatterned cross-section, so that a desired pattern can be created in atarget portion of the substrate. The terms “light valve” and “SpatialLight Modulator” (SLM) can also be used in this context. Examples ofsuch patterning means are provided below.

A programmable mirror array may comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that (for example) addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate spatial filter, the said undiffracted light can befiltered out of the reflected beam, leaving only the diffracted light toreach the substrate. In this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.It will be appreciated that, as an alternative, the filter may filterout the diffracted light, leaving the undiffracted light to reach thesubstrate.

An array of diffractive optical micro-electro-mechanical (MEMS) devicescan also be used in a corresponding manner. Each diffractive opticalMEMS device is comprised of a plurality of reflective ribbons that canbe deformed relative to one another to form a grating that reflectsincident light as diffracted light. A further alternative embodiment ofa programmable mirror array employs a matrix arrangement of tinymirrors, each of which can be individually tilted about an axis byapplying a suitable localized electric field, or by employingpiezoelectric actuation means. Once again, the mirrors arematrix-addressable, such that addressed mirrors will reflect an incomingradiation beam in a different direction to unaddressed mirrors. In thismanner, the reflected beam is patterned according to the addressingpattern of the matrix-addressable mirrors.

The required matrix addressing can be performed using suitableelectronic means. In both of the situations described hereabove, thearray of individually controllable elements can comprise one or moreprogrammable mirror arrays. More information on mirror arrays as herereferred to can be gleaned, for example, from U.S. Pat. No. 5,296,891and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 andWO 98/33096, which are incorporated herein by reference.

A programmable LCD array. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference.

It should be appreciated that where pre-biasing of features, opticalproximity correction features, phase variation techniques and multipleexposure techniques are used. For example, the pattern “displayed” onthe array of individually controllable elements may differ substantiallyfrom the pattern eventually transferred to a layer of or on thesubstrate. Similarly, the pattern eventually generated on the substratemay not correspond to the pattern formed at anyone instant on the arrayof individually controllable elements. This may be the case in anarrangement in which the eventual pattern formed on each part of thesubstrate is built up over a given period of time or a given number ofexposures during which the pattern on the array of individuallycontrollable elements and/or the relative position of the substratechanges.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat panel displays, thin-film magnetic heads, etc. The skilled artisanwill appreciate that, in the context of such alternative applications,any use of the terms “wafer” or “die” herein may be considered assynonymous with the more general terms “substrate” or “target portion,”respectively.

The substrate referred to herein may be processed, before or afterexposure, in for example a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist) or a metrologyor inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once, for example in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UY) radiation (e.g.,having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUY) radiation (e.g., having a wavelength in the range of20 nm), as well as particle beams, such as ion beams or electron beams.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens.”

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatusincludes an illumination system (illuminator) IL for providing aprojection beam PB of radiation (e.g., UV radiation) and an array ofindividually controllable elements PPM (e.g., a programmable mirrorarray) for applying a pattern to the projection beam. In general, theposition of the array of individually controllable elements will befixed relative to item PL. However, it may instead be connected to apositioning means for accurately positioning it with respect to the itemPL.

Also included is a substrate table (e.g., a wafer table) WT forsupporting a substrate (e.g., a resist-coated wafer) W, and connected topositioning means PW for accurately positioning the substrate withrespect to item PL. A projection system (“lens”) PL is provided forimaging a pattern imparted to the projection beam PB by the array ofindividually controllable elements PPM onto a target portion C (e.g.,comprising one or more dies) of the substrate W. The projection systemmay image the array of individually controllable elements onto thesubstrate. Alternatively, the projection system may image secondarysources for which the elements of the array of individually controllableelements act as shutters. The projection system may also comprise amicro lens array (known as an MLA), e.g., to form the secondary sourcesand to image microspots onto the substrate.

As depicted herein, the apparatus is of a reflective type (i.e., has areflective array of individually controllable elements). However, ingeneral, it may also be of a transmissive type, for example (i.e., witha transmissive array of individually controllable elements).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting means AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The beam PB subsequently intercepts the array of individuallycontrollable elements PPM. Having been reflected by the array ofindividually controllable elements PPM, the beam PB passes through theprojection system PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the positioning means PW (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the beam PE. Where used, the positioning means for the array ofindividually controllable elements can be used to accurately correct theposition of the array of individually controllable elements PPM withrespect to the path of the beam PB, e.g., during a scan.

In general, movement of the object table WT is realized with the aid ofa long-stroke module (course positioning) and a short-stroke module(fine positioning), which are not explicitly depicted in FIG. 1. Asimilar system may also be used to position the array of individuallycontrollable elements. It will be appreciated that the projection beammay alternatively/additionally be moveable while the object table and/orthe array of individually controllable elements may have a fixedposition to provide the required relative movement.

As a further alternative, that may be especially applicable in themanufacture of flat panel displays, the position of the substrate tableand the projection system may be fixed and the substrate may be arrangedto be moved relative to the substrate table. For example, the substratetable may be provided with a system for scanning the substrate across itat a substantially constant velocity.

Although the lithography apparatus according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and theapparatus may be used to project a patterned projection beam for use inresistless lithography.

The depicted apparatus can be used in four preferred modes. In a stepmode, the array of individually controllable elements imparts an entirepattern to the projection beam, which is projected onto a target portionC in one go (i.e., a single static exposure). The substrate table WT isthen shifted in the X and/or Y direction so that a different targetportion C can be exposed. In step mode, the maximum size of the exposurefield limits the size of the target portion C imaged in a single staticexposure.

In scan mode, the array of individually controllable elements is movablein a given direction (the so-called “scan direction”, e.g., the Ydirection) with a speed v, so that the projection beam PB is caused toscan over the array of individually controllable elements; concurrently,the substrate table WT is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the lensPL. In scan mode, the maximum size of the exposure field limits thewidth (in the non-scanning direction) of the target portion in a singledynamic exposure, whereas the length of the scanning motion determinesthe height (in the scanning direction) of the target portion.

In a pulse mode, the array of individually controllable elements is keptessentially stationary and the entire pattern is projected onto a targetportion C of the substrate using a pulsed radiation source. Thesubstrate table WT is moved with an essentially constant speed such thatthe projection beam PB is caused to scan a line across the substrate W.The pattern on the array of individually controllable elements isupdated as required between pulses of the radiation system and thepulses are timed such that successive target portions C are exposed atthe required locations on the substrate. Consequently, the projectionbeam can scan across the substrate W to expose the complete pattern fora strip of the substrate. The process is repeated until the completesubstrate has been exposed line by line.

A continuous scan mode is provided and is essentially the same as pulsemode. The exception is that in continuous scan mode, a substantiallyconstant radiation source is used and the pattern on the array ofindividually controllable elements is updated as the projection beamscans across the substrate and exposes it. Combinations and/orvariations on the above described modes of use or entirely differentmodes of use may also be employed.

FIG. 2 represents a portion of an apparatus according to the presentinvention. In particular, it shows an array of individually controllableelements 10, a system of projection elements 11 and an array of focusingelements 12 for directing radiation onto a substrate 13. It will beappreciated that an alternative arrangement for projecting the radiationfrom the array of individually controllable elements 10 onto the arrayof focusing elements 12 may also be used. It will also be appreciatedthat the array of individually controllable elements may be illuminatedby means of a beam splitter (arranged within the set of projectionelements such that it diverts the projection beam of radiation onto areflective array of individually controllable elements telecentrically,the reflection of which then passes straight through the beam splitterinto the remainder of the projection system as is well known), may beilluminated by oblique radiation (as shown in FIG. 2) or could also bedirectly illuminated if it is of a transmissive configuration.

As shown, the array of individually controllable elements 10 comprisesindividually controllable elements 21 to 26. The array of focusingelements 12 comprises two focusing elements 31, 32. The radiation fromthree of the individually controllable elements 21, 22, 23 is directedto one of the focusing elements 32 and radiation from the remainingindividually controllable elements 24, 25, 26 is directed to the otherfocusing element 31. It will be appreciated that, in practice, the arrayof focusing elements will have many more focusing elements. For example,the array of focusing elements may have 1025 by 968 focusing elements inthe array. Furthermore, it will be appreciated that the array ofindividually controllable elements will likewise be significantlylarger. Furthermore, as explained below, any number of individuallycontrollable elements may be associated with each of the focusingelements.

Each of the focusing elements 31, 32 focuses the radiation directed ontoit to an associated area 33, 34 on substrate 13. The intensity of theradiation in each area is dependant on the sum of the intensities of theportions of the patterned beam from each of the individuallycontrollable elements associated with that focusing element. Thereforethe radiation at the area 34 on the substrate 13 associated with thefocusing element 32 is dependant on the intensity of the radiationpropagating from each of individually controllable elements 21, 22, 23.Each of the individually controllable elements 21 to 26 can be set to aplurality of states. In a simple situation the elements may be set toeither direct radiation onto the associated focusing element 20 or notto. Therefore, it has two states, namely full intensity and zerointensity.

In the example shown in FIG. 2, being able to set each of the threeindividually controllable elements associated with each focusingelement, results in being able to provide four different levels ofradiation intensity at the area on the substrate to which the radiationfrom one focusing element is directed. Specifically, all of the elementsmay be set to zero radiation, resulting in zero radiation at the area onthe substrate illuminated by the focusing element. All the elements maybe set to full intensity, providing maximum intensity at the area on thesubstrate illuminated by the focusing element. Only one of theindividually controllable elements may be set to full intensity,providing an intensity level at the area on the substrate illuminated bythe focusing element that is one-third of the maximum intensity. Or twoof the individually controllable elements may be set to full intensity,providing two-thirds of the maximum intensity at the area on thesubstrate illuminated by the focusing element.

It will be appreciated that with different numbers of individuallycontrollable elements associated with each focusing element, differentnumbers of intensity levels or gray scales at the area on the substratemay be provided. With one individually controllable element per focusingelement, for example, two intensity levels are provided. With twoelements, three levels are provided. As discussed above with threeindividually controllable elements, four levels are provided, and so on.In the schematic representation shown in FIG. 2, the three individuallycontrollable elements associated with each focusing element are, forclarity, shown arranged in a row. It should be appreciated, however,that in practice the individually controllable elements may be arrangedin different configurations. For example, if four individuallycontrollable elements are used per focusing element, these may bearranged in a square configuration.

The preceding description has been in relation to the use ofindividually controllable elements that can either pass radiation to thefocusing elements or not. It will, be appreciated that in practice someradiation may be directed to the focusing element even in the lowintensity state of the individually controllable element. In otherwords, the two states of the individually controllable element will be arelatively higher intensity state and a relatively lower intensitystate.

The invention may also be used with individually controllable elementsthat can be set to additional states. For example, the elements may beset to one or more intermediate states in which radiation at anintensity between the high intensity level and the low intensity levelis directed to the focusing element. For example, each of theindividually controllable elements may be able to provide radiation atan intensity level half way between the higher intensity level and thelower intensity level.

In this case, the arrangement as shown in FIG. 2 would be able toprovide three additional levels of radiation intensity at the area onthe substrate illuminated by the focusing element, namely one betweenthe minimum intensity level and the one-third intensity level (asreferred to above), one between the one-third level and the two-thirdintensity level and one between the two-third intensity level and themaximum intensity level.

It will be appreciated that this benefit also applies to the use of anynumber of individually controllable elements associated with eachfocusing element. Furthermore, it will be apparent that providingadditional control states for each individually controllable elementfurther increases the number of intensity levels capable of beinggenerated at the area that is illuminated on the substrate. In practice,each individually controllable element may, for example, be capable ofgenerating up to 256 different intensity levels.

In a system as described above, there is some redundancy. For example,the radiation intensity at the area illuminated on the substrate will bethe same if a first of the individually controllable elements is set toa first state and the remaining two are set to a second state ascompared with a second of the individually controllable elements beingset to the first state and the remaining two being set to the secondstate.

Therefore, each of the individually controllable elements associatedwith one focusing element may be configured such that in each of theirstates they direct a different proportion of the intensity of theradiation that is incident on them to the associated focusing element.In this case, the intensity of the radiation received at the area on thesubstrate that is illuminated will be different if, for example, a firstof the individually controllable elements is set to its maximumintensity and the remaining individually controllable elements are setto minimum intensity then when a second of the individually controllableelements is set to its maximum intensity and the other elements are setto minimum intensity. Therefore, if as in this example, threeindividually controllable elements are used per focusing element andeach is capable of being set into three different states, nine differentintensity levels may be generated in the area of the substrate that isilluminated.

As described above, the individually controllable elements associatedwith each focusing element may be configured to transmit a differentproportion of the radiation incident on them to the focusing element ineach of their corresponding states. Alternatively, however, theapparatus may be configured such that the radiation that is incident oneach of the individually controllable elements associated with onefocusing element has a different intensity level. This may be achieved,for example, by providing an array of attenuators associated with thearray of individually controllable elements.

The incident radiation directed to a first one of the individuallycontrollable elements may, for example, not be attenuated at all whilethe radiation directed to each of the other individually controllableelements associated with the same focusing element are attenuated bydiffering amounts. Therefore, even though each of the individuallycontrollable elements directs the same proportion of the radiationincident upon it to the focusing element in each of the correspondingstates, the radiation received at the focusing element from each of theindividually controllable elements will be different.

Therefore, as before, additional intensity levels at the area of thesubstrate being illuminated by the focusing element are created. Insteadof attenuating the radiation that is incident on the individuallycontrollable elements, it will be appreciated that alternatively oradditionally, the radiation from each of the individually controllableelements may be attenuated between individually controllable elementsand the associated focusing elements.

The above described arrangements provide ways of controlling theintensity of the radiation in the area on the substrate that isilluminated by each of the focusing elements. Therefore, the radiationdose received by the areas can be varied when each of the areas on thesubstrate is illuminated for a given exposure time.

Further control of the radiation dose may be provided by arranging foreach area on the substrate to receive two exposures, one at each of twointensities. If, for example, each of the two exposures is for an equalamount of time, the dose received by the area will be the average of thedose that would have been received had the intensity level of the firstexposure been maintained for the complete exposure time and the dosethat would have been received had the intensity level of the secondexposure been maintained for the entire time. Therefore it is possibleprovide further intermediate radiation doses. It will be appreciatedthat by providing further exposures (i.e., more than two), furtherintermediate dose levels can be provided.

In practice, the substrate may be moved by an actuator relative to theprojection system at a constant velocity. In this case, the additionaldose control as discussed above may be provided for a point on thesubstrate by changing the settings applied to the individuallycontrollable elements while the point is passing through the areailluminated by the associated focusing element. For example, it may bechanged half way through the point's passage across the illuminatedarea.

Alternatively or additionally each point on the substrate may, as thesubstrate scans beneath the projection system, pass through the areailluminated by two or more different focusing elements. In this case thecontroller for setting the individually controllable elements may bearranged to provide the two different radiation intensity levels in thedifferent areas illuminated by the different focusing elements as thepoint passes through each of the illuminated areas.

Consequently, the point will be illuminated by each focusing element fora given amount of time at the requisite intensity level, producing theoverall radiation dose level required. In practice, for example, asingle point on the substrate may pass through several tens of differentareas illuminated by different focusing elements. Therefore, each pointpotentially receives several tens of independent exposures, permitting agreat number of gray tone levels to be generated.

It will be appreciated that any combination of the techniques discussedabove for controlling the radiation dose or gray tone level may be usedtogether. For example, a plurality of intensity levels or gray scalesmay be provided for each of a plurality of sub-exposures of each pointon the substrate.

FIGS. 3, 4 a, and 4 b illustrate the advantage of gray tone exposurecontrol. FIG. 3 shows a substrate 40 after exposure without gray tone.In areas 44, 45, a resist 42 has been completely removed, exposing thecorresponding portions of a device layer 41 which are then subject ofthe subsequent processing operations. In areas 43, 46, the full layer ofresist remains in place so the corresponding portions of the devicelayer underneath the resist are not affected by the subsequentprocessing operations.

FIG. 4 a shows the substrate 40 after an exposure using gray tone. Inaddition to areas 47, 50 where none of the resist has been removed andarea 49 where all of the resist have been removed, there is an area 48where the resist has been partially removed by a gray tone exposure(i.e., the area has received a dose of radiation between the minimum andmaximum dose). Accordingly in the processing steps that immediatelyfollow, only area 49 of the device layer 41 is exposed and affected bythe processing steps.

However, subsequently, as shown in FIG. 4 b, given thickness of resist42 is removed. This exposes the device layer 41 in area 48, which hadreceived the partial exposure (as well as the area 49 which is alreadyexposed), but does not expose the device layer in areas 47, 50 whichreceived the minimum exposure. In the process steps that follow,therefore, areas 48, 49 are affected but areas 47, 50 are not.

Accordingly the first set of process steps can be applied to a first setof areas and the second set of process steps can be applied to a secondset of areas after only a single radiation exposure step. Thus theprovision of gray tone control can be used to reduce the requirement forradiation exposure steps. It will be appreciated that by using largernumbers of gray tone levels and repeatedly removing a given uniformlevel of resist from the device between series of processing steps,further reduction in the number of radiation exposure steps can beachieved.

CONCLUSION

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

Any such alternate boundaries are thus within the scope and spirit ofthe claimed invention. One skilled in the art will recognize that thesefunctional building blocks can be implemented by analog and/or digitalcircuits, discrete components, application specific integrated circuits,firmware, processors executing appropriate software and the like or anycombination thereof. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. An apparatus comprising: a device configured to receive and pattern abeam of radiation; and an array of focusing elements comprising at leasttwo focus elements, each being optically associated with a first portionand a second portion of the device; wherein the first portion of thedevice receives a first portion of the beam of radiation and the secondportion of the device receives a second portion of the beam ofradiation, the first portion of the beam of radiation having a differentintensity than the second portion of the beam of radiation, wherein thefirst portion of the device provides its corresponding focus elementwith the first portion of the beam of radiation and the second portionof the device provides its corresponding focus element with the secondportion of the beam of radiation.
 2. The apparatus of claim 1, whereinthe first and second portions of the device each comprise an array ofindividually controllable elements.
 3. The apparatus of claim 2, whereinthe array of individually controllable elements comprises a plurality ofindividually controllable elements arranged in a square configuration.4. The apparatus of claim 2, wherein the array of individuallycontrollable elements comprises a programmable mirror array.
 5. Theapparatus of claim 1, wherein the array of focusing elements comprisesmore than two focus elements.
 6. A method, comprising: receiving, by afirst portion of a device, a first portion of a beam of radiation, andby a second portion of the device, a second portion of the beam ofradiation, wherein the first portion of the beam of radiation has adifferent intensity than the second portion of the beam of radiation;patterning the first portion of the beam of radiation to generate afirst portion of a patterned beam and the second portion of the beam ofradiation to generate a second portion of the patterned beam; focusing,using a first focusing element, the first portion of the patterned beamand, using a second focusing element, the second portion of thepatterned beam; and exposing a substrate with the first and secondportions of the patterned beam after the first and second potions of thepatterned beam are focused using the first and second focus elements. 7.The method of claim 6, wherein the patterning comprises patterning thebeam of radiation with a plurality of individually controllableelements.
 8. The method of claim 7, wherein the patterning with theplurality of individually controllable elements comprises patterning thebeam of radiation with a programmable mirror array.
 9. The method ofclaim 7, wherein the focusing comprises receiving, at the first focuselement, the first portion of the patterned beam and, at the secondfocus element, the second portion of the patterned beam.